The human heart has four cardiac valves: the mitral valve, the tricuspid valve, the pulmonary valve and the aortic valve. The mitral valve is situated in the left atrio-ventricular ostium and regulates the unidirectionality of the flow of blood from the atrium to the ventricle. It opens in the diastole and closes in the systole, preventing blood from flowing back from the ventricle to the atrium. The annulus of a normally functioning mitral valve is characterized by shape, dimensions and flexibility such as to allow a correct closure of the valve lips during the systolic phase. For example, the mitral annulus has a characteristic “kidney” shape (or “D” shape), and is more flexible in the portion corresponding to the posteria lip of the valve. Illnesses or genetic defects can cause deformations or dilatations of the annulus of the mitral valve, resulting in an incomplete closure thereof with consequent regurgitation of blood. The same phenomena can occur in the tricuspid valve, situated between the right atrium and right ventricle.
A frequently used method for eliminating some pathological alterations of the mitral and tricuspid valves is that of reinstating the correct shape and dimensions of the value annulus by means of surgical procedures known as annuloplasty. Annuloplasty comprises surgically implanting a supporting prosthesis on the dilated or deformed annulus for the purpose of reinstating its dimensions and/or physiological shape in such a way as to allow the cardiac valve to function correctly.
Support prostheses utilized in valve repair operations are called annuloplasty prostheses. In the majority of cases such prostheses are constituted by a closed or open ring structure comprising an inner core and an outer cladding of biocompatible material which allows surgical suture.
Annuloplasty prostheses of various types have been described in the prior art. Initially, the prostheses proposed were predominantly of the rigid type for the purpose of drastically reducing the dilatation of the valve annulus. Such prostheses are generally constituted by a metal core (for example, a titanium alloy), an optional sheath of cladding around the core, and an outer cladding of textile for suturing. Rigid annuloplasty prostheses are described, for example, in U.S. Pat. No. 4,055,861 by Carpentier et al., issued Nov. 1, 1977, and U.S. Pat. No. 3,656,185 by Carpentier et al., issued Apr. 18, 1972.
Rigid prostheses, although satisfactory as far as reinstatement of the shape and dimensions of the valve annulus are concerned, do not allow the annulus of the valve to flex along the base of the posterior cuspid in such a way as to assist the cardiac muscle movements. Consequently, significant stress is imposed on the suture points subjected to torsion and traction, which prevents natural behavior of the valve.
Subsequently, semi-rigid or completely flexible prosthesis models were proposed. Completely flexible annuloplasty prostheses are described, for example, in U.S. Pat. No. 5,041,130 by Carpentier et al., issued Aug. 20, 1991, U.S. Pat. No. 5,716,397 by Myers et al., issued Feb. 10, 1998, U.S. Pat. No. 6,102,945 by Campbell et al., issued Aug. 15, 2000, and U.S. Pat. No. 5,064,431 by Gilbertson et al., issued Nov. 12, 1991. The completely flexible prostheses follow the movements of the annulus during the cardiac cycle in an optimal manner. However, they have the disadvantage of not allowing the shape to be reconstructed in an optimal manner.
Semi-rigid prostheses seek to unite the advantages of the rigid type with those of the completely flexible type while avoiding the disadvantages of each. Semi-rigid annuloplasty prostheses are described, for example, in U.S. Pat. Nos. 5,061,277 by Carpentier et al., issued Oct. 29, 1991, U.S. Pat. No. 5,104,407 by Lam et al., issued Apr. 14, 1992, U.S. Pat. No. 5,674,279 by Wright et al., issued Oct. 7, 1997, U.S. Pat. No. 5,824,066 by Gross et al., issued Oct. 20, 1998, U.S. Pat. No. 5,607,471 by Seguin et al., issued Mar. 4, 1997, and U.S. Pat. No. 6,143,024 by Campbell et al., issued Nov. 7, 2000.
In particular, U.S. Pat. No. 5,104,407 describes a ring prosthesis comprising an annular support element which is substantially more rigid in one part than in the remainder. This more rigid part projects transversely with respect to the general plane in which the remaining part of the support element lies. On the other hand, U.S. Pat. No. 5,607,471 describes a ring prosthesis having variable transverse sections, and therefore differentiated rigidity along its circumferential extent.
All these known prostheses have, however, limitations in relation to the possibility of exhibiting a variable rigidity in dependence on the point and/or direction and/or mode of application of the stresses. For example, U.S. Pat. No. 5,104,407 provides a structure with variable rigidity in the plane of the ring, but with a limited possibility of flexure outside this plane. On the other hand, U.S. Pat. No. 5,607,471 describes technical arrangements able to guarantee an improved flexibility out of the plane of the annulus. However, the possibility of effectively obtaining characteristics of variable rigidity of the prosthesis, for example, in its localised portions as well, appears significantly limited by the necessity to vary the area of the entire resistant section of the internal element. Moreover, the technical arrangements described in U.S. Pat. No. 5,607,471 do not allow an optimal behavior of the prosthesis in response to stresses in a tangential direction to be obtained (that is to say stresses directed along its longitudinal axis, which is usually closed in a ring), the rigidity in response to traction and compression at the same point of the internal element being substantially undifferentiated.